1. Field of the Invention
The present invention relates to a developer, a developer cartridge, and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus by electrophotography comprises a photosensitive body, a charging device, an exposing device, a developing device, a transferring device, and a fixing device.
In the image forming apparatus, the surface of an image drum (a photosensitive drum) of the photosensitive body is equally, uniformly charged by a charging roller, and then, exposed by an LED head so that an electrostatic latent image is formed. In the developing process, a developer or toner, is adhered to the electrostatic latent image by the developing device to form a toner image. The toner image is transferred onto a print medium or paper by a transferring roller. The print paper carrying the toner image is fed to the fixing device so that the toner image is fused (fixed) on the paper. The developing device comprises a pair of fusing rollers to press the heated and melted toner of the toner image.
In order to provide a satisfactory fusion in the fixing device, it is necessary that the toner be easy to be melted. Also, a parting agent is added to the toner to easily part the toner from the fusing roller. Many of the parting agents have the property of being melted more easily than the resin contained in the toner.
Thus, it is possible to make the toner to be melted more easily by using this property, thereby increasing the fusing characteristic. Also, it is possible to prevent a hot offset phenomenon that the melted toner in the fixing device is adhered to the fusing roller. That is, the parting agent secures an offset margin.
A synthetic wax, such as polyethylene or polypropylene, or a natural wax, such as carnauba, is added alone or in combination to the resin, a main component of the toner. It is well known that a softener, such as a fatty acid ester, has a similar parting property and is used as the parting agent.
When an OHP sheet is used, the image forming apparatus of a color electrophotographic system, such as a color printer or a color copying machine requires a high OHP transparency in contrast to the image forming apparatus of a monochrome system. Consequently, the toner is required to be melted more readily than before.
A fluidizing agent (hereinafter “additive agents”) is usually added to toner particles containing crystal resin (hereinafter “developer primary particle”) to reduce the viscosity and increase the fluidization of the toner. Examples of the additive agent includes an inorganic abradant, such as a silicon oxide (silica), a surface-treated silica, titanium, a titanium oxide, a surface-treated titanium oxide, a clay, alumina, and calcium carbonate, and an organic abradant, such as a methacrylic resin abradant, a melanin resin abradant, and silicone resin abradant.
The particle diameter of the additive agent is smaller that that of the developer primary particle, that is, 2–5,000 nm, generally 5–2,000 nm.
In the manufacturing process of the toner, an adding apparatus, such as a Henschel mixer, is used for adhering the additive agent to the surfaces of the developer primary particles to make the finished toner.
In the conventional image forming apparatus, however, printing endurance tests show that a developing blade filming and/or an image drum filming occurred due to the adhesion of the toner and/or additive agent to the surfaces of a developing blade and/or an image drum, when the used toner has a viscosity coefficient of 1×104 (poise) at 105° C. measured by a flow tester and one part by weight of R972 (made by Nippon Aerosil Co., Ltd.) and one part by weight of RX50 (made by Nippon Aerosil Co., Ltd.) as the additive agent).
This developing blade filming occurred in printing a few hundreds of sheets in contrast to the usual blade filming that occurs in printing as many as a few thousands to a few tens of thousands of sheets. Especially in the color printing, photo-pictures and/or poster pictures of a high printing duty are continuously printed so that the developing blade filming occurs more frequently. In addition, when a toner having a low viscosity coefficient is used, the developing blade filming occurs even for a document having a low printing duty.
The probability of the image drum filming is slightly lower than that of the developing blade filming. However, when a toner having a low viscosity coefficient when melted and photo-pictures and/or poster pictures are continuously printed, the image drum filming occurs.